I live near O'Hare airport and see hundreds of planes every day. This evening a plane flew past that did not have the standard red - green wing tip lights. It was white only on both sides. It caught my attention because of that, and because the plane was unusually quiet...not silent, but quieter than most everything accept the smallest private planes. Any suggestions as to what I saw?
You don't tell us if you're viewing this aircraft from the front (approaching you) or the back (moving away from you), so let's look at all the options:
Aircraft have three colors of steady-burning navigation/position lights: Red on the left (port) side, Green on the right (starboard) side, and White at the aftmost position (tail). This diagram shows the required lighting coverage (as implemented on a typical small aircraft):
(With the exception of a "mast light" these are identical to naval vessel position lights - the function of a mast light is performed by the landing light, though it's not used as a "position light" the way the others are.)
On some aircraft there will also be aft-facing white position lights on the wingtips, combined with the navigation and strobe lights:
The aft-facing position lights on the wingtip improve visibility at night, particularly flying over brightly lit urban areas (a problem sailing vessels don't have). They also give a visual reference to determine the size (wingspan) of the aircraft.
Finally (just to throw another wrench in the works) some aircraft are equipped with wingtip "recognition lights" (forward-facing white lights on the wingtip):
These are typically used on the ground or in approach to landing, but when illuminated the brighter recognition lights can make the forward position lights difficult to see, particularly from the ground where your eyes probably aren't adapted for darkness.
So putting it all together:
If you're viewing the aircraft from the rear (it's flying away from you) you will only see white lights (either one or three white lights) and the aircraft's strobes or other intermittent anti-collision lights. You will thus know the aircraft is traveling away from you.
If you're viewing the aircraft from the front (it's flying directly toward you) you will see a red and green light (possibly with white taxi/landing/recognition lights as well, but the red and green at the wingtips are what you're looking for to identify the aircraft's direction of travel).
Again if the aircraft has wingtip "recognition lights" you may have trouble picking out the navigation lights.
If you're viewing the aircraft from the side (it's flying perpendicular to you) you will see either the Red or Green light until it passes in front of you, at which point one (or more) of the white lights will come into view.